Studies are proposed which examine three modes of modulation of the signal (calcium binding) which stimulates heart myofibrils to shorten and develop force. Covalent modulation will be studied by measuring covalent phosphate content of cardiac myofibrillar thin filaments (troponin I) and thick filaments (myosin P light chain) during responses to stimulatory, inhibitory and pathological perturbations of in situ and isolated beating hearts. Levels of phosphorylation determined above will be mimicked with synthetic contracto-regulatory complexes and measurements of myofibrillar function made. Non-covalent modulation will be studies by determination of ion selectivity profiles of myofibrillar calcium receptors and cooperative phenomona within the myofibrillar lattice affecting myofibrillar calcium binding. Linkage dependent modulation will be studied by measurement of thin filament calcium receptor (troponin C) calcium binding in the presence of varying concentrations of connected cross-bridges.